A. Field of Invention
This invention relates to methods and apparatuses for drawing wire and more specifically to methods and apparatuses for controlling the speed and torque of the drawing blocks used on a wire drawing machine.
B. Description of the Related Art
It is know to draw wire through multiple dies at multiple die positions of a wire drawing machine. The hardness of drawn steel wire results from the plastic deformation associated with the drawing process. The wire increases in hardness as it proceeds through the wire drawing machine. If the wire becomes too hard or brittle, breakage occurs during the drawing process or when the wire is subjected to torsion or bending.
As the wire is drawn through a die to reduce the cross sectional area, the outer fibers of the wire flow faster or at a higher velocity than those in its center. This flow velocity variation causes a lesser amount of elongation at the center of the wire than at the surface of the wire. A stress differential resulting from this mechanism of elongation induces compressive, longitudinal stresses on the surface of the wire and tensile, longitudinal stresses at its center. Voids, known as central bursts, can occur in the center of the wire when the tensile stresses exceed the breaking strength of the material. The central burst effect can be prevented by controlling the process geometries.
Strain introduced into the wire by the drawing process increases the tensile strength of the wire. Preferably, this increase is held constant at every die of the draft in a wire drawing machine. Analyses of the formation of central bursts show that bursting is more likely to occur if the increase in tensile strength remains low. Therefore, the wire is generally drawn through a draft of many dies each having a geometry to avoid the central burst zone.
Generally a drawing capstan or drawing block is used juxtaposed to each die in order to draw or pull the wire through the corresponding die. A well-known problem in the field of wire drawing machines is the problem of slip. Slip is the difference in the speed of the wire versus the tangential speed of the drawing block that the wire is traveling on. If slip is not properly controlled, both wire properties as well as the wire drawing machine can be damaged.
It is well-known to provide a drive system for use in rotating the drawing blocks of a wire drawing machine. Typically, one motor is used to drive multiple drawing blocks. Such a design generally works well for its intended purpose. The problem, however, with this type of drive system is that slip at each drawing block cannot be carefully controlled. One attempted solution to this problem is to provide one motor for each drawing block. In this way, the speed and torque at each drawing block can be carefully controlled thereby controlling slip. The problem with this solution, however, is the expense of providing all the required motors. It is well-known, for example, to use twenty (20) or more drawing blocks which would require twenty (20) or more motors.
What is needed is a method and device to accurately adjust the rotational condition of each drawing block without the need for multiple motors.